The relationship between genomes and epigenomes reveals how cells with identical DNA develop different ______ blank

Choice D is the best answer. The convention being tested is the use of a colon within a sentence. In this choice, the colon correctly introduces the following explanation of the different functions of genomes and epigenomes.

Choice A is incorrect because it results in a run-on sentence. The two main clauses ("The relationship…functions" and "whereas…expressed") are fused without punctuation and/or a conjunction. Choice B is incorrect because it results in a comma splice. A comma can’t be used in this way to join two main clauses ("The relationship…functions" and "whereas…expressed"). Choice C is incorrect. Without a comma preceding it, the conjunction "and" can’t be used in this way to join two main clauses. Furthermore, "and" fails to indie that what follows is an explanation of how cells with identical DNA develop different functions.